But the celebrity bear died at the Berlin Zoo Saturday, in front of hundreds of visitors, the Associated Press reported.

The sad news has sparked a wave of tributes online, with fans around the world taking to Twitter and Facebook to memorialize the famous bear. But it has also sparked curiosity among followers about why Knut died so young when polar bears are expected to live 15 to 20 years, or longer.

Polar Bears in Captivity Can Live Longer Than 30 Years, Scientist Says

Heiner Kloes, Knut's keeper at the Berlin Zoo, told the Associated Press, "He was by himself in his compound, he was in the water, and then he was dead. ...He was not sick, we don't know why he died."

Scientists say it's impossible to know what led to Knut's death until after the postmortem, which zoo officials said will take place Monday. They acknowledged that it is unusual for polar bears to die in captivity at such a young age.

"It's a little bit surprising," said Peter Ewins, an arctic species specialist with the World Wildlife Fund. "In captivity, polar bears can live longer than in the wild; to 25 or 30. Even more than 30 years old because they're not exposed to the elements and hard realities of life in the wild."

For polar bears in the wild, the first real hurdle is birth, and some animals continue to struggle in their first year or two of life. But, at 4, Knut would have been like an 18-year-old human, Ewins said.

"He'd be coming into breeding age," he said. "He's supposed to have been in his prime."

Psychological Trauma, Parasites Can Lead to Death for Animals in Captivity

While Ewins said he is unfamiliar with conditions at the Berlin Zoo, one of several causes can contribute to the death of animals in captivity.

"One is the psychological trauma, because they're wild animals and he was born in a zoo," he said. "Just the effects of being in captivity, it's more of neurological, psychological thing that affects some animals."

For some animals living in zoos, parasites and diseases can also lead to death, he said, but emphasized, "I would have thought that the Berlin Zoo is pretty vigilant on that."

He also said that physiological complications could have led to Knut's death, as well as "the remote possibility" that the animal ingested a sharp object thrown into his pen by a zoo visitor.

It's possible that Knut's early separation from his mother may have also played a role in his death," he said.

"Maybe his immune system or some other bodily function wasn't working right because he was fed on some kind of formula," Ewins said.

Regardless of what led to Knut's sudden death, fans around the world are bidding adieu to their beloved bear.